Tribology is the general term that refers to design and operating dynamics of the bearing-lubrication-rotor support structure of machinery. Several tribology techniques can be used for predictive maintenance: lubricating oil analysis, spectrographic analy- sis, ferrography, and wear particle analysis. Lubricating oil analysis, as the name implies, is an analysis technique that determines the condition of lubricating oils used in mechanical and electrical equipment. It is not a tool for determining the operating condition of machinery. Some forms of lubricat- ing oil analysis will provide an accurate quantitative breakdown of individual chem- ical elements, both oil additive and contaminates, contained in the oil. A comparison of the amount of trace metals in successive oil samples can indicate wear patterns of oil-wetted parts in plant equipment and will provide an indication of impending machine failure. Until recently, tribology analysis has been a relatively slow and expensive process. Analyses were conducted using traditional laboratory techniques and required exten- sive, skilled labor. Microprocessor-based systems are now available that can automate most of the lubricating oil and spectrographic analysis, thus reducing the manual effort and cost of analysis. The primary applications for spectrographic or lubricating oil analysis are quality control, reduction of lubricating oil inventories, and determination of the most cost- effective interval for oil change. Lubricating, hydraulic, and dielectric oils can be peri- odically analyzed using these techniques, to determine their condition. The results of this analysis can be used to determine if the oil meets the lubricating requirements of the machine or application. Based on the results of the analysis, lubricants can be changed or upgraded to meet the specific operating requirements. In addition, detailed analysis of the chemical and physical properties of different oils used in the plant can, in some cases, allow consolidation or reduction of the number 9 TRIBOLOGY 202 and types of lubricants required to maintain plant equipment. Elimination of unnec- essary duplication can reduce required inventory levels and therefore maintenance costs. As a predictive maintenance tool, lubricating oil and spectrographic analysis can be used to schedule oil change intervals based on the actual condition of the oil. In mid- size to large plants, a reduction in the number of oil changes can amount to a con- siderable annual reduction in maintenance costs. Relatively inexpensive sampling and testing can show when the oil in a machine has reached a point that warrants change. The full benefit of oil analysis can only be achieved by taking frequent samples and trending the data for each machine in the plant. It can provide a wealth of informa- tion on which to base maintenance decisions; however, major payback is rarely pos- sible without a consistent program of sampling. 9.1 LUBRICATING OIL ANALYSIS Oil analysis has become an important aid to preventive maintenance. Laboratories rec- ommend that samples of machine lubricant be taken at scheduled intervals to deter- mine the condition of the lubricating film that is critical to machine-train operation. 9.1.1 Oil Analysis Tests Typically, the following tests are conducted on lube oil samples: Viscosity Viscosity is one of the most important properties of lubricating oil. The actual vis- cosity of oil samples is compared to an unused sample to determine the thinning or thickening of the sample during use. Excessively low viscosity will reduce the oil film strength, weakening its ability to prevent metal-to-metal contact. Excessively high vis- cosity may impede the flow of oil to vital locations in the bearing support structure, reducing its ability to lubricate. Contamination Contamination of oil by water or coolant can cause major problems in a lubricating system. Many of the additives now used in formulating lubricants contain the same elements that are used in coolant additives. Therefore, the laboratory must have an accurate analysis of new oil for comparison. Fuel Dilution Dilution of oil in an engine, caused by fuel contamination, weakens the oil film strength, sealing ability, and detergency. Improper operation, fuel system leaks, Tribology 203 ignition problems, improper timing, or other deficiencies may cause it. Fuel dilution is considered excessive when it reaches a level of 2.5 to 5 percent. Solids Content The amount of solids in the oil sample is a general test. All solid materials in the oil are measured as a percentage of the sample volume or weight. The presence of solids in a lubricating system can significantly increase the wear on lubricated parts. Any unexpected rise in reported solids is cause for concern. Fuel Soot Soot caused by the combustion of fuels is an important indicator for oil used in diesel engines and is always present to some extent. A test to measure fuel soot in diesel engine oil is important because it indicates the fuel-burning efficiency of the engine. Most tests for fuel soot are conducted by infrared analysis. Oxidation Oxidation of lubricating oil can result in lacquer deposits, metal corrosion, or oil thick- ening. Most lubricants contain oxidation inhibitors; however, when additives are used up, oxidation of the oil begins. The quantity of oxidation in an oil sample is measured by differential infrared analysis. Nitration Nitration results from fuel combustion in engines. The products formed are highly acidic, and they may leave deposits in combustion areas. Nitration will accelerate oil oxidation. Infrared analysis is used to detect and measure nitration products. Total Acid Number (TAN) The acidity of the oil is a measure of the amount of acid or acid-like material in the oil sample. Because new oils contain additives that affect the TAN, it is important to compare used oil samples with new, unused oil of the same type. Regular analysis at specific intervals is important to this evaluation. Total Base Number (TBN) The base number indicates the ability of oil to neutralize acidity. The higher the TBN, the greater its ability to neutralize acidity. Typical causes of low TBN include using the improper oil for an application, waiting too long between oil changes, overheat- ing, and using high-sulfur fuel. 204 An Introduction to Predictive Maintenance Particle Count Particle count tests are important to anticipating potential system or machine prob- lems. This is especially true in hydraulic systems. The particle count analysis made as a part of a normal lube oil analysis is different from wear particle analysis. In this test, high particle counts indicate that machinery may be wearing abnormally or that failures may occur because of temporarily or permanently blocked orifices. No attempt is made to determine the wear patterns, size, and other factors that would identify the failure mode within the machine. Spectrographic Analysis Spectrographic analysis allows accurate, rapid measurements of many of the elements present in lubricating oil. These elements are generally classified as wear metals, contaminants, or additives. Some elements can be listed in more than one of these classifications. Standard lubricating oil analysis does not attempt to deter- mine the specific failure modes of developing machine-train problems. Therefore, additional techniques must be used as part of a comprehensive predictive maintenance program. 9.1.2 Wear Particle Analysis Wear particle analysis is related to oil analysis only in that the particles to be studied are collected by drawing a sample of lubricating oil. Whereas lubricating oil analysis determines the actual condition of the oil sample, wear particle analysis provides direct information about the wearing condition of the machine-train. Parti- cles in the lubricant of a machine can provide significant information about the machine’s condition. This information is derived from the study of particle shape, composition, size, and quantity. Wear particle analysis is normally conducted in two stages. The first method used for wear particle analysis is routine monitoring and trending of the solids content of machine lubricant. In simple terms, the quantity, composition, and size of particulate matter in the lubricating oil indicates the machine’s mechani- cal condition. A normal machine will contain low levels of solids with a size less than 10 microns. As the machine’s condition degrades, the number and size of particulate matter increases. The second wear particle method involves analysis of the particu- late matter in each lubricating oil sample. Types of Wear Five basic types of wear can be identified according to the classification of particles: rubbing wear, cutting wear, rolling fatigue wear, combined rolling and sliding wear, and severe sliding wear. Only rubbing wear and early rolling fatigue mechanisms gen- erate particles that are predominantly less than 15 microns in size. Tribology 205 Rubbing Wear. Rubbing wear is the result of normal sliding wear in a machine. During a normal break-in of a wear surface, a unique layer is formed at the surface. As long as this layer is stable, the surface wears normally. If the layer is removed faster than it is generated, the wear rate increases and the maximum particle size increases. Exces- sive quantities of contaminant in a lubrication system can increase rubbing wear by more than an order of magnitude without completely removing the shear mixed layer. Although catastrophic failure is unlikely, these machines can wear out rapidly. Impending trouble is indicated by a dramatic increase in wear particles. Cutting Wear Particles. Cutting wear particles are generated when one surface pene- trates another. These particles are produced when a misaligned or fractured hard surface produces an edge that cuts into a softer surface, or when abrasive contaminant becomes embedded in a soft surface and cuts an opposing surface. Cutting wear particles are abnormal and are always worthy of attention. If they are only a few microns long and a fraction of a micron wide, the cause is probably contamination. Increasing quantities of longer particles signals a potentially imminent component failure. Rolling Fatigue. Rolling fatigue is associated primarily with rolling contact bearings and may produce three distinct particle types: fatigue spall particles, spherical particles, and laminar particles. Fatigue spall particles are the actual material removed when a pit or spall opens up on a bearing surface. An increase in the quantity or size of these particles is the first indication of an abnormality. Rolling fatigue does not always gen- erate spherical particles, and they may be generated by other sources. Their presence is important in that they are detectable before any actual spalling occurs. Laminar par- ticles are very thin and are formed by the passage of a wear particle through a rolling contact. They often have holes in them. Laminar particles may be generated through- out the life of a bearing, but at the onset of fatigue spalling the quantity increases. Combined Rolling and Sliding Wear. Combined rolling and sliding wear results from the moving contact of surfaces in gear systems. These larger particles result from tensile stresses on the gear surface, causing the fatigue cracks to spread deeper into the gear tooth before pitting. Gear fatigue cracks do not generate spheres. Scuffing of gears is caused by too high a load or speed. The excessive heat generated by this con- dition breaks down the lubricating film and causes adhesion of the mating gear teeth. As the wear surfaces become rougher, the wear rate increases. Once started, scuffing usually affects each gear tooth. Severe Sliding Wear. Excessive loads or heat causes severe sliding wear in a gear system. Under these conditions, large particles break away from the wear surfaces, causing an increase in the wear rate. If the stresses applied to the surface are increased further, a second transition point is reached. The surface breaks down, and catastrophic wear enses. Normal spectrographic analysis is limited to particulate contamination with a size of 10 microns or less. Larger contaminants are ignored. This fact can limit the benefits derived from the technique. 206 An Introduction to Predictive Maintenance 9.1.3 Ferrography This technique is similar to spectrography, but there are two major exceptions. First, ferrography separates particulate contamination by using a magnetic field rather than by burning a sample as in spectrographic analysis. Because a magnetic field is used to separate contaminants, this technique is primarily limited to ferrous or magnetic particles. The second difference is that particulate contamination larger than 10 microns can be separated and analyzed. Normal ferrographic analysis will capture particles up to 100 microns in size and provides a better representation of the total oil contamination than spectrographic techniques. 9.1.4 Oil Analysis Costs and Uses There are three major limitations with using tribology analysis in a predictive main- tenance program: equipment costs, acquiring accurate oil samples, and interpretation of data. The capital cost of spectrographic analysis instrumentation is normally too high to justify in-plant testing. The typical cost for a microprocessor-based spectrographic system is between $30,000 and $60,000; therefore, most predictive maintenance pro- grams rely on third-party analysis of oil samples. Simple lubricating oil analysis by a testing laboratory will range from about $20 to $50 per sample. Standard analysis normally includes viscosity, flash point, total in- solubles, total acid number (TAN), total base number (TBN), fuel content, and water content. More detailed analysis, using spectrographic or ferrographic techniques, that includes metal scans, particle distribution (size), and other data can cost more than $150 per sample. A more severe limiting factor with any method of oil analysis is acquiring accurate samples of the true lubricating oil inventory in a machine. Sampling is not a matter of opening a port somewhere in the oil line and catching a pint sample. Extreme care must be taken to acquire samples that truly represent the lubricant that will pass through the machine’s bearings. One recent example is an attempt to acquire oil samples from a bullgear compressor. The lubricating oil filter had a sample port on the clean (i.e., downstream) side; however, comparison of samples taken at this point and one taken directly from the compressor’s oil reservoir indicated that more conta- minants existed downstream from the filter than in the reservoir. Which location actu- ally represented the oil’s condition? Neither sample was truly representative. The oil filter had removed most of the suspended solids (i.e., metals and other insolubles) and was therefore not representative of the actual condition. The reservoir sample was not representative because most of the suspended solids had settled out in the sump. Proper methods and frequency of sampling lubricating oil are critical to all predictive maintenance techniques that use lubricant samples. Sample points that are consistent Tribology 207 with the objective of detecting large particles should be chosen. In a recirculating system, samples should be drawn as the lubricant returns to the reservoir and before any filtration occurs. Do not draw oil from the bottom of a sump where large quanti- ties of material build up over time. Return lines are preferable to reservoir as the sample source, but good reservoir samples can be obtained if careful, consistent prac- tices are used. Even equipment with high levels of filtration can be effectively mon- itored as long as samples are drawn before oil enters the filters. Sampling techniques involve taking samples under uniform operating conditions. Samples should not be taken more than 30 minutes after the equipment has been shut down. Sample frequency is a function of the mean time to failure from the onset of an abnor- mal wear mode to catastrophic failure. For machines in critical service, sampling every 25 hours of operation is appropriate; however, for most industrial equipment in con- tinuous service, monthly sampling is adequate. The exception to monthly sampling is machines with extreme loads. In this instance, weekly sampling is recommended. Understanding the meaning of analysis results is perhaps the most serious limiting factor. Results are usually expressed in terms that are totally foreign to plant engi- neers or technicians. Therefore, it is difficult for them to understand the true meaning of results, in terms of oil or machine condition. A good background in quantitative and qualitative chemistry is beneficial. At a minimum, plant staff will require train- ing in basic chemistry and specific instruction on interpreting tribology results. 9.2 SETTING UPANEFFECTIVE PROGRAM Many plants have implemented oil analysis programs to better manage their equip- ment and lubricant assets. Although some have received only marginal benefits, a few have reported substantial savings, cost reductions, and increased productivity. Success in an oil analysis program requires a dedicated commitment to understanding the equipment design, the lubricant, the operating environment, and the relationship between test results and the actions to be performed. In North America, millions of dollars have been invested in oil analysis programs with little or no financial return. The analyses performed by original equipment manufac- turers or lubricant manufacturers are often termed as “free.” In many of these cases, the results from the testing have little or no effect on the maintenance, planning, and/or evaluated equipment’s condition. The reason is not because this service is free, or the ability of the laboratory, or the effort of the lubricant supplier to provide value-added service. The reason is a lack of knowledge—a failure to understand the value lost when a sample is not representative of the system, and the inability to turn equipment and lubricant data into useful information that guides maintenance activities. More important is the failure to understand the true requirements and operating char- acteristics of the equipment. This dilemma is not restricted to the companies receiv- ing “free” analysis. In many cases, unsuccessful or ineffective oil analysis programs are in the same predicament. Conflicting information from equipment suppliers, 208 An Introduction to Predictive Maintenance laboratories, and lubricant manufacturers have clouded the true requirements of equipment to the maintenance personnel or individuals responsible for the program. The following steps provide a guideline to implementing an effective lubricating oil analysis program. 9.2.1 Equipment Audit An equipment audit should be performed to obtain knowledge of the equipment, its internal design, the system design, and the present operating and environmental con- ditions. Failure to gain a full understanding of the equipment’s operating needs and conditions undermines the technology. This information is used as a reference to set equipment targets and limits, while supplying direction for future maintenance tasks. The information should be stored under an equipment-specific listing and made acces- sible to other predictive technologies, such as vibration analysis. Equipment Criticality Safety, environmental concerns, historical problems, reliability, downtime costs, and repairs must all be considered when determining the equipment to be included in a viable lubricating oil analysis program. Criticality should also be the dominant factor used to determine the frequency and type of analyses that will be used to monitor plant equipment and systems. Equipment Component and System Identification Collecting, categorizing, and evaluating all design and operating manuals including schematics are required to understand the complexity of modern equipment. Original equipment manufacturers’ assistance in identifying the original bearings, wear sur- faces, and component metallurgy will take the guesswork out of setting targets and limits. This information, found in the operating and maintenance manuals furnished with each system, will aid in future troubleshooting. Equipment nameplate data with accurate model and serial numbers allow for easy identification by the manufacturer to aid in obtaining this information. Care should be exercised in this part of the evaluation. In many cases, critical plant systems and equipment has been modified one or more times over their installed life. Information obtained from operating and maintenance manuals or directly from the original equipment manufacturer must be adjusted to reflect the actual installed equipment. Operating Parameters Equipment designers and operating manuals reflect the minimum requirements for operating the equipment. These include operating temperature, lubricant requirements, pressures, duty cycles, filtration requirements, and other parameters that directly or indirectly impact reliability and life-cycle cost. Operating outside these parameters will adversely impact equipment reliability and the lubricant’s ability to provide Tribology 209 adequate protection. It may also require modifications and/or additions to the system to allow the component to run within an acceptable range. Operating Equipment Evaluation A visual inspection of the equipment is required to examine and record the compo- nents used in the system, including filtration, breathers, coolers, heaters, and so on. This inspection should also record all operating temperatures and pressures, duty cycles, rotational direction, rotating speeds, filter indicators, and the like. Tempera- ture reading of the major components is required to reflect the component operating system temperature. A noncontract, infrared scanner may be used to obtain accurate temperature readings. Operating Environment Hostile environments or environmental contamination is usually not considered when the original equipment manufacturer establishes equipment operating parameters. These conditions can influence lubricant degradation, eventually resulting in damaged equipment. All environmental conditions such as mean temperature, humidity, and all possible contaminants must be recorded. Maintenance History Reliable history relating to wear and lubrication-related failures can assist in the decision-making process of adjusting and tightening targets and limits. These targets should allow for advanced warnings of historical problems and possible root-cause detection. Oil Sampling Location A sampling location should be identified for each piece of equipment to allow for trouble-free, repetitive, and representative sampling of the health of the equipment and the lubricant. This sampling method should allow the equipment to be tested under its actual operating condition while being unobtrusive and safe for the technician. New Oil Baseline A sample of the new lubricant is required to provide a baseline or reference point for physical and chemical properties of the lubricant. Lubricants and additive packages can change over time, so adjusting lubrication targets and alarms should reflect these changes. Cooling Water Baseline A sample of the cooling water, when used, should be collected, tested, and ana- lyzed to obtain its physical and chemical properties. These results are used to 210 An Introduction to Predictive Maintenance adjust the lubricant targets and to reflect and provide early warnings of leaks in the coolers. Targets and Alarms Original equipment manufacturing (OEM) operating specifications or the guidelines of a recognized governing body can be used in setting the minimum alarms. These alarms must be set considering all of the previously collected information. These set- tings must provide early detection of contaminants, lubricant deterioration, and present equipment health. These achievable targets should be set to supply an early warning of any anomalies that allow corrective actions to be planned, scheduled, and performed with little or no effect on production schedules. Database Development A database should be developed to organize equipment information and the collected data along with the equipment-specific targets and alarms. This database should be easy to use. The end user must have control of the targets and limits in order to reflect the true equipment-specific conditions within the plant. In ideal circumstances, the database should be integrated into a larger predictive main- tenance database that contains all information and data that are useful to the predic- tive maintenance analysts. Combining vibration, lubricating oil, infrared, and other predictive data into a single database will greatly enhance the analysts’ ability to detect and correct incipient problems and will ensure that maximum benefits are obtained from the program. 9.2.2 Lubricant Audit Process Equipment reliability requires a lubricant that meets and maintains specific physical, chemical, and cleanliness requirements. A detailed trail of a lubricant is required, beginning with the oil supplier and ending after disposal of spent lubricants. Sampling and testing of the lubricants are important to validate the lubricant condition through- out its life cycle. Lubricant Requirements Information from the equipment audit supplies the physical and chemical requirements of the lubricant to operate within the equipment. After ensuring that the correct type of lubricant is in use, the audit information ensures that the correct viscosity is used in relationship to the true operating temperature. Lubricant Supplier Quality control programs implemented by the lubricant manufacturer should be questioned and recorded when evaluating the supplier. Sampling and testing new Tribology 211 [...]... for the direction of any required maintenance activities, which will ensure safe, reliable, and cost-effective operation of the plant equipment Routine Monitoring Routine monitoring is designed to collect the required data to competently inform the predictive maintenance analysts or maintenance group of the present condition of its 214 An Introduction to Predictive Maintenance lubricants and equipment... and operating results gained by predictive maintenance into a value and benefits in the financial terms necessary to ensure continued management support Without credible financial links to the facility and organization’s business objectives, technical criteria are essentially 216 An Introduction to Predictive Maintenance useless As a result, many successful predictive maintenance programs are being curtailed... that can affect its production capacity Typical systems include heat exchangers, pumps, filtration, boilers, fans, blowers, and other critical systems 2 17 218 An Introduction to Predictive Maintenance Inclusion of process parameters in a predictive maintenance program can be accomplished in two ways: manual or microprocessor-based systems Both methods normally require installing instrumentation to measure... Evaluation Predictive maintenance tasks are based on condition measurements and performance on the basis of defects before outright failure impacts safety and production Wellmanaged predictive maintenance programs are capable of identifying and tracking anomalies Success is often measured by factors such as number of machines monitored, problems recognized, number of saves, and other technical criteria Few maintenance. .. is caused by fatigue The only positive way to prevent or minimize these failures is to ensure that proper maintenance is performed regularly on these components It is important to follow the manufacturer’s recommendations for valve maintenance and replacement 224 An Introduction to Predictive Maintenance Table 10–3 Common Failure Modes of Reciprocating Positive-Displacement Pumps ᭹ Abrasives or Corrosives... that will be used in your program In addition, some of the microprocessor-based predictive maintenance systems can calculate unknown process variables For example, they can calculate the pump efficiency used in the example This ability to calculate unknowns based on measured variables will enhance a totalplant predictive maintenance program without increasing the manual effort required In addition, some... critical to plant operation Vibration-based predictive maintenance will provide the mechanical condition of the pump, and infrared imaging will provide the condition of the electric motor and bearings Neither provides any indication of the operating efficiency of the pump Therefore, the pump can be operating at less than 50 percent efficiency and the predictive maintenance program would not detect the problem...212 An Introduction to Predictive Maintenance lubricants before dispensing ensures that the vendor has supplied the correct lubricant Oil Storage Correct labeling, including materials safety display system (MSDS), must be... Design Unsuitable Pumps in Parallel Operation ᭹ ᭹ ᭹ ᭹ Viscosity Too High ᭹ ᭹ Wrong Rotation ᭹ Source: Integrated Systems, Inc ᭹ ᭹ ᭹ Speed Too High ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ ᭹ 220 An Introduction to Predictive Maintenance Cavitation Cavitation in a centrifugal pump, which has a significant, negative effect on performance, is the most common failure mode Cavitation not only degrades a pump’s performance... required maintenance Symptoms of Changed Conditions The symptoms of failure caused by variations in TSH include changes in motor speed and flowrate Motor Speed The brake horsepower of the motor that drives a pump is load dependent As the pump’s operating point deviates from BEP, the amount of horsepower required also changes This causes a change in the pump’s rotating speed, 222 An Introduction to Predictive . equipment suppliers, 208 An Introduction to Predictive Maintenance laboratories, and lubricant manufacturers have clouded the true requirements of equipment to the maintenance personnel or individuals. Evaluation Predictive maintenance tasks are based on condition measurements and performance on the basis of defects before outright failure impacts safety and production. Well- managed predictive maintenance. a properly imple- mented program. 216 An Introduction to Predictive Maintenance Many plants do not consider machine or systems efficiency as part of the maintenance responsibility; however, machinery